The aerodynamic principles of airfoils have been the subject of continuing study since the mid-1800's. Scientists, engineers and experimentalists have continually sought to understand and improve on the aerodynamic characteristics of the airfoil. Much of this development has been spurred by the growing interest in flying and the desire to build a safer aircraft.
The primary concern with any airfoil design is two-fold: first, to produce a greater amount of lift without detrimentally increasing drag and, second, to enable the airfoil to function at greater angles of attack without stalling. Coupling these desired parameters with the wide range of airspeeds to which the airfoil may be exposed results in a multitude of airfoil designs, each with its own aerodynamic characteristics to perform optimally at a specific flight condition. With respect to the wing of an airplane, for example, a design suitable for producing substantial lift at low airspeeds inherently produces excessive drag at high airspeeds. On the other hand, a wing designed to fly with minimal drag at high airspeeds generally fails to produce sufficient lift at low airspeeds to maintain flight, as during takeoffs and landings. This latter condition results in a stalling of the wing as the angle of attack of the wing is increased, in an effort to produce greater lift, until the critical angle of attack is exceeded. It is recognized, of course, that an airfoil will stall at any airspeed whenever the angle of attack of the airfoil to the free stream airflow exceeds the critical angle of attack for the particular airfoil.
In an effort to improve the overall aerodynamic characteristics of the airplane wing throughout a wide variety of flight conditions, designers have turned to movable slots and/or flaps on the leading and trailing edges of the wing which change the cross-sectional profile of the wing. These slots and/or flaps may be adjusted during flight for optimum performance of the wing at various flight conditions. For example, at high airspeeds, the slots and/or flaps are fully retracted to give the wing a relatively thin, streamlined profile thereby reducing the drag acting thereon. At lower airspeeds, however, the slots and/or flaps are extended downward to produce a greater camber on the wing which permits the wing to develop greater lift, albeit with greater drag. Such use of slots and/or flaps, therefore, increase the aircraft's operational angles of attack, i.e., the angles of attack through which the aircraft can safely operate. However, because of the increase in drag, the extension of slots and/or flaps is only advisable at relatively low airspeeds and, thus, they are unable to improve the lift and stall characteristics of the wing at high, cruising airspeeds.
Efforts have been made to design an airfoil having improved stall characteristics at high airspeeds. One of the most interesting designs is an airfoil having a substantially wedge-shaped profile with a step-like discontinuity on the under surface thereof. While this design exhibits improved stall characteristics, tests have shown that such is accomplished at the expense of lift and with substantial reductions in the lift to drag ratio of the airfoil.
Despite the extensive work conducted in this field, no airfoil, as yet, has been developed which provides improved stall characteristics at virtually all operational airspeeds while simultaneously providing improved lift and lift to drag ratios.